free will?

You brought up a good point that in some cases, believing the idea of free will is comforting, and in other scenarios, not so much. If no one believed in free will, parents would feel more responsible for their own children's faults. They would see no point in teaching them to be different. 

But if most people believed in free will, then it would be detrimental to people like drug addicts, students trying to learn, and just about anyone trying to better themselves. A drug addict would be able to justify their addiction by saying that he or she is not responsible for their own actions, that it's just because of genes. A student might fail a test and instead of developing a growth mindset to work hard and study for the next one, the student would accept their fate that they're not cut out to be smart. In both cases, the lack of free will would help people justify and accept their own actions. Since there is no incentive to change, they wouldn't even try. 

Whether or not there is free will, I think it's interesting to think about the psychological impact that such a belief system could have on everyone.